Glare Effect Transperancy Problem
There is nothing wrong with your file. Blender's viewer is broken, and has been broken for many years.

It cannot display pixels that are luminous and transparent (it cannot do proper Associated alpha). Switch to Color (RGB) view instead of color and Alpha. You should be able to see the halos and blurs.

As an alternative composite over a black background.

Beware that if you try to save the image as png with alpha you will get no halos or blurs, as the format doesn't support associated alpha either.



More details on these links:

Glow effect invisible on transparent background

How do I get the Glare node to output transparent instead of Black background

Render semi-transparent volume with cycles